Edible coatings, which are defined as thin layers of wax or other substances applied to the surface of food, have been employed for over 800 years to increase the shelf life of food. In the United States, wax coatings have been utilized commercially since 1930s, when oranges were coated with melted paraffin waxes. These early coatings were used to produce the appearance of a glossy skin. In recent years, coatings have been used to preserve attributes connected with fruit and vegetable quality and affording shelf-life extension involving a decrease in weight loss and respiration rate while providing glossy appearance and the possible prevention of damage induced by insect penetration (Nussinovitch in Modern biopolymer science: bridging the divide between fundamental treatise and industrial application, Kasapis et al. (Eds.), New York Academic Press, Elsevier Inc., 295-326, 2009).
Pepper fruit (Capsicum annuum L.) is naturally hollow and is therefore characterized by limited water reservoir capacity. Accordingly, the loss of small amounts of water may result in loss of freshness and firmness, a reduction in fruit quality, shelf life and market value (Maalekuu et al., J. Am. Soc. Horticult. Sci., 130, 735-741, 2005). The major factor shortening the postharvest life of bell peppers is water loss (Maalekuu et al., Adv. Horticult. Sci., 17, 187-191, 2003). Once the fruit is harvested, its tissue may rapidly dehydrate since the water potential (Ψ), which quantifies the water content of the surrounding air, is much lower than that of the plant tissue thus causing the diffusion of water from the fruit tissue to the environment. Consequently, postharvest shriveling of the fruit occurs (Dijkink et al., Postharvest Biol. Technol., 32, 311-320, 2004). Flaccidity, shriveling, wilting and decay are major problems that decrease marketability and consumer acceptance of postharvest bell pepper fruit. Flaccidity is also directly associated with the loss of water during storage when respiration as well as diffusion of water through the cuticle occur. Shriveling and wilting are processes which are evident in water loss of 5% or more. Hence, reduction of water loss, especially through diffusion through the cuticle, would help maintain textural quality and external appearance of the fruit thus improving its storage life.
Achieving water-saturated atmosphere around the fruit by individual-seal, shrink-wrap or modified-atmosphere packaging (MAP) has been employed. Bell peppers individually wrapped in plastic film showed marked reduction in weight loss and softening, which resulted in an extended shelf-life. Although individual-seal and MAP appeared to reduce bell pepper fruit moisture loss, a number of limitations inhibited commercial use. One limitation that has been encountered is the development of aerobic microorganisms due to water condensation caused by temperature fluctuations during storage or transportation. It was noted that film wrapping increased the incidence of bacterial soft rot in bell pepper compared to non-wrapped peppers. It was also shown that shrink-wrapped pepper developed higher populations of total aerobic microorganisms, yeasts, and molds as compared to non-wrapped peppers. Another limitation involved environmental apprehensions about the use of plastic materials. Hence, replacement of plastic films with edible or biodegradable materials is attractive from an environmental perspective.
The most common methods used nowadays for reducing water loss of postharvest fruit include lowering the temperature and/or raising the relative humidity (RH) of the storage environment. However, these storage environments can cause chilling injury, enhance disease development and increased incidence of fruit decay. In general, it is relatively difficult to preserve the quality of postharvest bell peppers as compared to other fruits, due to the peppers' sensitivity to low temperatures (<7° C.), water loss and rot development (Meir et al., Postharvest Biol. and Technol., 5, 303-309 1995).
The application of edible coatings and films in fruits and vegetables has received awareness worldwide for improvement of postharvest life (Lerdthanangkul et al., J. Food Sci., 61, 176-179, 1996; Conforti et al., Food Chem. Toxicol., 67, 1360-1363, 2002; Ozden et al., Euro. Food Res. Technol., 214, 320-326, 2002; Ayranci et al., Food Chem., 87, 339-342, 2004; Beaulieu et al., Indust. Crops and Products, 30, 271-275, 2009; Sabularse et al., Int. J. Food Sci. Nutri., 60, 206-218, 2009). However, despite their advantages, edible coatings for fruits can also adversely affect their quality. For example, an edible coating used to reduce the rate of water loss might interfere with fruit's respiration, resulting in off-flavors (Park, Trends Food Sci. Technol., 10, 254-260, 1999; Chen et al., Food Hydrocolloids, 14, 561-568, 2000). In addition, the surface gloss of food is a very important parameter since it reflects on its quality in the eye of the consumer. Bell peppers have very high natural gloss levels compared to other fruits (Nussinovitch et al., Lebensmittel-Wissenschaft und-Technologie, 29, 184-186, 1995). When very glossy surfaces, such as those of red bell pepper, are treated with available coating formulations, their natural shine may be diminished.
U.S. Pat. Nos. 6,299,915 and 6,068,867 to one of the inventors of the present invention disclose a hydrocolloid protective coating for food and/or agricultural products comprising dried hydrocolloid gel, one or more natural compounds isolated from the surface of said product or a compound substantially equivalent thereto and other optional additives. The protective coating provides improved protection of the product, thereby extending its shelf-life.
U.S. Patent application No. 2004/0146617 and U.S. Pat. No. 7,222,455 disclose methods for suppressing cracking, stem browning, and water loss in fruit or vegetables, such as cherries. The methods comprise applying to fruit or vegetables an amount of a wax emulsion comprising a matrix of complex hydrocarbons, one or more emulsifying agents, and water. In some embodiments, the wax emulsion comprises from about 0.125% to about 25% (weight/weight) of carnauba wax, from about 0.1% to about 16% (weight/weight) of oleic acid, and from about 0.03% to about 6% (weight/weight) of morpholine, and from about 53% to about 99.7% (weight/weight) of water.
Edible coatings on fruits can serve as gas or moisture barriers. They can help diminish moisture loss, and/or reduce fruit oxygen uptake from the environment and thus slow respiration. Edible coatings have been reported to be effective on various kinds of fruits and vegetables (U.S. Pat. Nos. 7,771,763 and 7,169,423). Chitosan coating reduced weight loss, respiration rate, loss of color, wilting, and fungal infection of bell pepper during storage at 13° C. and 20° C. at 85% RH (El Ghaouth et al., Journal of Food Processing and Preservation, 15 (5), 359-368, 1991). Although several attempts have been made to design edible coatings for bell pepper fruit, two main problems remained unsolved. First, the coating provided only ˜4% reduction in the weight loss of the coated commodity and second, the coating reduced the natural gloss of the pepper.
There remains an unmet need in the art for an edible coating of bell pepper which provides an extension of its shelf life without impairing the natural gloss and taste thereof.